Novak Djokovic described his return to the world number one ranking as a "phenomenal achievement" as he and old rival Roger Federer coasted into the Paris Masters quarterfinals.
Four-time Paris champion Djokovic made the last-eight when Bosnian opponent Damir Dzumhur retired injured trailing 1-6, 1-2 on Thursday.
Federer defeated Italy's Fabio Fognini 6-4, 6-3 as the Swiss great moved three wins away from his 100th career title.
Djokovic will replace Rafael Nadal as the world's top player on Monday after a two-year absence.
"Reflecting on what I've been through in the last year, it's quite a phenomenal achievement," said Djokovic who will face Marin Cilic for a spot in the semifinals.
"I'm very, very happy and proud about it. Five months ago, it was highly improbable considering my ranking and the way I played and felt on the court."
Federer, 37, who captured his 99th trophy on home ground in Basel last week, will next face Japan's Kei Nishikori who beat South Africa's Kevin Anderson 6-4, 6-4.
Nishikori, seeded 10, had lost to seventh-seeded Anderson in the Vienna final on Sunday.
However, he has a 2-6 record against Federer with his most recent win over the 20-time major champion coming in 2014.
Third seed Federer is playing the Paris Masters for the first time since 2015 and is bidding to clinch the tournament for just the second time after winning it in 2011.
Djokovic was guaranteed a return to the ranking summit when Nadal withdrew from Paris with an abdominal injury on Wednesday.
Cilic, the fifth seed, secured his quarterfinal ticket with a 7-6 (7/5), 6-4 defeat of Grigor Dimitrov.
Cilic fired 29 winners to advance to the last eight for the third straight year.
The win also improved the Croat's prospects of making the ATP Finals in London starting on November 11.
"I am very close to London, to securing my spot mathematically," said Cilic, who is part of the Croatia team facing France in the Davis Cup final at the end of this month.
Cilic has beaten Djokovic just twice in 17 meetings.
Karen Khachanov saved two match points against John Isner on his way to defeating the eighth-seeded American 6-4, 6-7 (9/11), 7-6 (10/8).
The Russian will next play Alexander Zverev after the German fourth seed beat Diego Schwartzman of Argentina 6-4, 6-2.
Austrian sixth seed Dominic Thiem reached the quarterfinals in Paris for the first time, seeing off in-form Borna Coric 6-7 (3/7), 6-2, 7-5.
Thiem, also in contention for the ATP Finals, next faces defending champion Jack Sock of the United States.
